The Clinical Decision Systems Research Resource is engaged in the development, evaluation, and implementation of a computer-based diagnostic system for internal medicine, referred to as the INTERNIST system. This resource provides users with on-line access to an extensive medical data base encompassing more than 500 diseases and three thousand manifestations of disease, which can be used for educational purposes as well as diagnostic consultation. The diagnostic system itself is based on a heuristic 'problem-formation' procedure which has demonstrated capability for solving complex clinical problems involving multiple diseases.